A key unit is one type of components constituting mobile equipment such as portable phones and personal digital assistants (PDA) or the like, and a multiplicity of keys (push buttons) for switch operation are assembled and arrayed on a surface of a single sheet in one unit. A single key comprises a key top made of a hard resin adhered to the surface of flexible keypads made of various rubbers such as a silicone rubber, a thermoplastic elastomer or the like, and a switch-pressing projection (what is called “pressing element”) formed on the back surface of the keypad corresponding to the position of the key top. When a circuit substrate provided with switch elements close contacts with the undersurface of the key unit configured in such manner, key switches are formed at the positions corresponding to each key.
However, the conventional rubber keypads are too soft and lack shape stability, which causes various disadvantages such as incapability of precisely maintaining gaps between key tops and the difficulty of the suitability for automatic assembly. In particular, when a casing is provided with a key frame in order to maintain gaps between the keys, such a new disadvantage occurs that the key tops do not return due to interference with the key frame.
In order to overcome these weak points, there is conceived of the configuration of adding a hard plate material (hereinafter, referred to as “reinforcing plate”) to the keypad, and the present applicant has filed a patent application with the title of “hard base key unit” (Patent Reference 1).
In the hard base key unit, the usage of a rubber elastic body is limited only to a key operation part in a keypad, and other parts of the keypad are replaced with a hard resin plate for reinforcement. An aperture is provided at the position corresponding to the key operation part of the hard resin plate which serves as a reinforcing plate, and a rubber elastic body is jointed around the aperture. Further, there are also some alternative conditions where the rubber elastic body covers the overall top surface of the hard resin plate together with the aperture, or the rubber elastic body enfolds the overall hard resin plate including the apertures.
However, the role of the reinforcing plate in Patent Reference 1 only remains imparting rigidity to the keypads and serving also as a light-conducting plate, and no idea is recognized such that the reinforcing plate is positively utilized for a design element of equipment.
Patent Reference 1: Japanese Unexamined Patent Publication No. 2003-178639